LED calculator online. Calculation and selection of resistance for an LED

For stable operation, an LED requires a constant voltage source and a stabilized current, which will not exceed the values ​​​​permissible by the specifics of a particular LED. If you need to connect indicator LEDs, the operating current of which does not exceed 50-100 mA, you can limit the current using resistors. If we are talking about powering powerful LEDs with operating currents from hundreds of milliamps to a few amperes, then you cannot do without special devices - drivers (read more about these devices in the article "Drivers for LEDs", ready-made driver models can be seen.). Next, we will consider options when the required current is small and resistors can still be used.

Resistors are passive elements - they simply limit the current, but do not stabilize it in any way. The current will change with the voltage according to Ohm's law. The current is limited by a resistor by the banal conversion of “excess” electricity into heat according to the formula

P = I 2 R, where P is the heat generated in watts, I is the current in the circuit in amperes, R is the resistance in ohms.

The device naturally heats up. A resistor's ability to dissipate heat is not unlimited and, if the permissible current is exceeded, it will burn out. The permissible power dissipation is determined by the resistor body. This must be taken into account when planning the connection of LEDs and select elements with at least a double safety margin.

If you need to connect one LED, then the resistance of the resistor can be calculated, in accordance with Ohm's law, using a simple formula:

R = (U - U L) / I, where R is the required resistance in ohms, U is the power supply voltage, U L is the voltage drop across the LED in volts, I is the required LED current in amperes.

Very often you need to connect not one, but several LEDs. In this case, they can be connected in series or in parallel.

The voltage drop across LEDs connected in series is added up, and the same current flows through each of them. The power supply voltage must be greater than the total voltage drop.

The resistance of the resistor is calculated according to the same principle as in the case of one LED, only the voltage drop is taken into account not on one firefly, but in total for the entire chain.

Serial connection is convenient because it requires a minimum of additional parts, in addition, a large current is not required from the power source. But with a large number of LEDs, significant voltage may be required. In addition, if one of the sequential chain burns out, the chain will break and all LEDs will stop shining. Also, with this connection option, it is important to use exactly the same LEDs, otherwise their different parameters will serve as a source of imbalance. As a result, they can either shine unevenly or fail much faster.

Parallel connection is equivalent to simultaneous connection of individual LEDs, which do not “need to know” about the presence of other LEDs. In this case, the voltage of the power supply must exceed the voltage drop across one LED. The current strength of each LED can be adjusted individually by selecting the resistance of the resistor connected to it. It is important that the power supply "knows" how many LEDs are connected to it, since the total current it will be required to provide is equal to the sum of the currents flowing through all the LEDs. If one of the LEDs fails, nothing will happen to the glow of the others, since they work individually. Please note that this does not apply to parallel LEDs that are powered by a current limiting driver! The driver stabilizes the current; failure of one of the branches will lead to a general decrease in current. The driver immediately compensates for this reduction, which will lead to an increase in current on the remaining branches. And they may not survive it. For a similar reason, you should avoid connecting multiple parallel LEDs through a single current-limiting resistor.

The resistance of each resistor when connecting LEDs in parallel is calculated, I repeat, in the same way as when connecting one LED.

Parallel connection of LEDs does not require a high supply voltage, but when using it, it is necessary to provide sufficient current. More parts are required, but LEDs with different parameters can be connected simultaneously. Also, more current-limiting resistors, which will generate heat, will result in lower overall circuit efficiency compared to a series connection.

An LED is a semiconductor device with a nonlinear current-voltage characteristic (volt-ampere characteristic). Its stable operation, first of all, depends on the magnitude of the current flowing through it. Any, even minor, overload leads to degradation of the LED chip and a decrease in its working life.

To limit the current flowing through the LED at the desired level, the electrical circuit must be supplemented with a stabilizer. The simplest current-limiting element is a resistor.

Important! The resistor limits but does not stabilize the current.

Calculating a resistor for an LED is not a difficult task and is done using a simple school formula. But it is recommended to take a closer look at the physical processes occurring in the p-n junction of an LED.

Theory

Mathematical calculation

Below is a circuit diagram in its simplest form. In it, the LED and resistor form a series circuit through which the same current (I) flows. The circuit is powered by an EMF voltage source (U). In operating mode, a voltage drop occurs across the circuit elements: across the resistor (U R) and across the LED (U LED). Using Kirchhoff's second rule, we obtain the following equality: or its interpretation

In the above formulas, R is the resistance of the calculated resistor (Ohm), R LED is the differential resistance of the LED (Ohm), U is the voltage (V).

The R LED value changes as the operating conditions of the semiconductor device change. In this case, the variable quantities are current and voltage, the ratio of which determines the value of resistance. A clear explanation of this is the current-voltage characteristic of the LED. In the initial section of the characteristic (up to approximately 2 volts), a smooth increase in current occurs, as a result of which R LED is of great importance. Then the pn junction opens, which is accompanied by a sharp increase in current with a slight increase in the applied voltage.

By simply transforming the first two formulas, you can determine the resistance of the current-limiting resistor: U LED is the nameplate value for each individual type of LED.

Graphic calculation

Having the current-voltage characteristic of the LED under study in hand, you can calculate the resistor graphically. Of course, this method does not have wide practical application. After all, knowing the load current, you can easily calculate the value of the forward voltage from the graph. To do this, it is enough to draw a straight line from the ordinate axis (I) until it intersects with the curve, and then lower the line to the abscissa axis (U LED). As a result, all the data for calculating the resistance have been obtained.

However, the graph option is unique and deserves some attention.

Let's calculate a resistor for an LED with a rated current of 20 mA, which must be connected to a 5 V power source. To do this, draw a straight line from the 20 mA point until it intersects with the LED curve. Next, through the 5 V point and the point on the graph, draw a line until it intersects with the ordinate axis and obtain the maximum current value (I max), approximately equal to 50 mA. Using Ohm's law, we calculate the resistance: For the circuit to be safe and reliable, it is necessary to prevent overheating of the resistor. To do this, find its dissipation power using the formula:

In what cases is it possible to connect an LED through a resistor?

You can connect an LED through a resistor if the issue of circuit efficiency is not paramount. For example, using an LED as an indicator to illuminate a switch or mains voltage indicator in electrical appliances. In such devices, brightness is not important, and power consumption does not exceed 0.1 W. When connecting an LED with a consumption of more than 1 W, you need to be sure that the power supply produces a stabilized voltage.

If the input voltage of the circuit is not stabilized, then all noise and surges will be transmitted to the load, disrupting the operation of the LED. A striking example is the automotive electrical network, in which the voltage on the battery is only theoretically 12 V. In the simplest case, LED lighting in the car should be done through a linear stabilizer from the LM78XX series. And in order to somehow increase the efficiency of the circuit, you need to turn on 3 LEDs in series. Also, a power supply circuit through a resistor is in demand for laboratory purposes for testing new LED models. In other cases, it is recommended to use a current stabilizer (driver). Especially when the cost of the emitting diode is comparable to the cost of the driver. You receive a ready-made device with known parameters, which just needs to be connected correctly.

Examples of calculations of resistance and power of a resistor

To help beginners get their bearings, here are a couple of practical examples of calculating resistance for LEDs.

Cree XM-L T6

In the first case, we will calculate the resistor required to connect a powerful LED to a voltage source of 5 V. Cree XM–L with bin T6 has the following parameters: typical U LED = 2.9 V and maximum U LED = 3.5 V at current I LED =0.7 A. The typical value of U LED should be substituted into the calculations, since. it most often corresponds to reality. The calculated resistor value is present in the E24 series and has a tolerance of 5%. However, in practice it is often necessary to round the results to the nearest value from the standard series. It turns out that, taking into account rounding and a tolerance of 5%, the real resistance changes and, following it, the current changes in inverse proportion. Therefore, in order not to exceed the operating load current, it is necessary to round the calculated resistance upward.

Using the most common resistors from the E24 series, it is not always possible to select the desired value. There are two ways to solve this problem. The first involves the sequential inclusion of an additional current-limiting resistance, which should compensate for the missing Ohms. Its selection must be accompanied by control current measurements.

The second method provides higher accuracy, as it involves installing a precision resistor. This is an element whose resistance does not depend on temperature and other external factors and has a deviation of no more than 1% (series E96). In any case, it is better to leave the actual current slightly less than the nominal value. This will not greatly affect the brightness, but will provide the crystal with a gentle operating mode.

The power dissipated by the resistor will be:

The calculated resistor power for the LED must be increased by 20–30%.

Let's calculate the efficiency of the assembled lamp:

Example with LED SMD 5050

By analogy with the first example, we will figure out what resistor is needed for. Here you need to take into account the design features of the LED, which consists of three independent crystals.

If the LED SMD 5050 is single-color, then the forward voltage in the open state on each crystal will differ by no more than 0.1 V. This means that the LED can be powered from one resistor, combining 3 anodes into one group, and three cathodes into another. Let's select a resistor for connecting a white SMD 5050 with the following parameters: typical U LED = 3.3 V at a current of one chip I LED = 0.02 A. The closest standard value is 30 Ohms.

We accept for installation a limiting resistor with a power of 0.25 W and a resistance of 30 Ohms ±5%.

The SMD 5050 RGB LED has a different forward voltage for each die. Therefore, you will have to control the red, green and blue colors with three resistors of different values.

Online calculator

The online calculator for LEDs presented below is a convenient addition that will perform all the calculations independently. With its help, you don’t have to draw or calculate anything manually. All you need is to enter the two main parameters of the LED, indicate their number and the voltage of the power source. With one click of the mouse, the program will independently calculate the resistance of the resistor, select its value from the standard range and indicate the color marking. In addition, the program will offer a ready-made switching circuit.

Any LED has low resistance. If you connect it directly to the power supply, it will immediately burn out because the current is too high. The wires that connect it to the external terminals are made of copper or gold and cannot withstand a surge in current. That is why it is important to correctly calculate the resistor for the LED.

How long this LED will work depends on the correctness of the calculation. If the resistor has insufficient resistance, the LED may burn out, but if, on the contrary, the current is less than the rated current, the light bulb will have a dim light. In order to carry out calculations, there are special formulas and it is not difficult to do. In addition, there are special programs that will automatically make all the necessary calculations based on the entered data.

This article will discuss all aspects and subtleties of making such calculations. Also, as a bonus, the article contains a video on this topic and a scientific article that can be downloaded.

Calculation result

As a rule, it will turn out that resistors with this value are not produced, and you will be shown the nearest standard value. If you cannot make an accurate selection of resistance, then use a larger value. A suitable value can be made by connecting the resistance in parallel or in series. You don't have to calculate the resistance for an LED if you use a powerful variable or trimming resistor. The most common type is 3296 at 0.5W. When using a 12V power supply, up to 3 LEDs can be connected in series.

Resistors come in different accuracy classes, 10%, 5%, 1%. That is, their resistance can vary within these limits in a positive or negative direction. Do not forget to take into account the power of the current-limiting resistor, this is its ability to dissipate a certain amount of heat. If it is small, it will overheat and fail, thereby breaking the electrical circuit. To determine the polarity, you can apply a small voltage or use the diode test function on a multimeter. Different from resistance measurement mode, usually supplied from 2V to 3V.

Also, when calculating LEDs, you should take into account the spread of parameters; for cheap ones they will be maximum, for expensive ones they will be more the same. To check this parameter, you need to enable them under equal conditions, that is, sequentially.

By reducing the current or voltage, reduce the brightness to slightly glowing points. Visually, you will be able to estimate that some will glow brighter, others dimly. The more evenly they burn, the less spread. The LED resistor calculator assumes that the characteristics of the LED chips are ideal, that is, the difference is zero.

The drop voltage for common low-power models up to 10W can be from 2V to 12V. As power increases, the number of crystals in a COB diode increases; each has a drop. The crystals are connected in chains in series, then they are combined into parallel circuits. At powers from 10W to 100W the reduction increases from 12V to 36V. This parameter must be indicated in the technical characteristics of the LED chip and depends on the purpose of the color:

• blue;
• red;
• green;
• yellow;
• three-color RGB;
• four-color RGBW;
• two-color;
• warm and cool white.

Before choosing a resistor for an LED using an online calculator, you should make sure of the parameters of the diodes. The Chinese sell a lot of LEDs on Aliexpress, passing them off as branded ones. The most popular models are SMD3014, SMD 3528, SMD2835, SMD 5050, SMD5630, SMD5730. For example, most often the Chinese cheat on SMD5630 and SMD5730. The numbers in the markings only indicate the case size of 5.6mm by 3.0mm.

In branded ones, such a large case is used to install powerful 0.5W crystals, so buyers of SMD5630 diodes directly associate it with 0.5W power. The cunning Chinese takes advantage of this and installs a cheap and weak crystal in the 5630 case with an average power of 0.1W, while indicating the energy consumption of 0.5W.

A good example would be car lamps and LED corn lamps, which contain a large number of weak and low-quality LED chips. The average buyer believes that the more LEDs, the better the light and the higher the power. Car lamps on the weakest ice 0.1W To save money, my LED colleagues are looking for decent LEDs on Aliexpress. They look for a good seller who promises certain parameters, order, and wait a month for delivery. After tests, it turns out that the Chinese seller cheated and sold junk. You'll be lucky if the seventh time you get decent diodes and not junk. Usually they make 5 orders, and without achieving results, they go to place an order in a domestic store that can make an exchange.

Calculating an LED Resistor Using Ohm's Law

Ohm's law states that the resistance of a resistor is R = V / I, where V = voltage through the resistor (V = S – V L in this case), I = current through the resistor. So R = (V S – V L) / I. If you want to connect several LEDs at once, this can be done in series. This reduces energy consumption and allows you to connect a large number of diodes at the same time, for example, as some kind of garland. All LEDs that are connected in series must be of the same type. The power supply must have sufficient power and provide the appropriate voltage.

It will be interesting➡ What is a thermistor?

Calculation example: Red, yellow and green diodes - when connected in series, a supply voltage of at least 8V is required, so a 9-volt battery will be an almost ideal source. V L = 2V + 2V + 2V = 6V (three diodes, their voltages are summed). If the supply voltage V S is 9 V and the diode current = 0.015A, Resistor R = (V S – V L) / I = (9 – 6) /0.015 = 200 Ohms. We take a 220 Ohm resistor (the nearest standard value, which is larger).

Avoid connecting LEDs in parallel!

LED as a nonlinear element

Let's consider a family of current-voltage characteristics (CV characteristics) for LEDs of various colors. This characteristic shows the dependence of the current passing through the light-emitting diode on the voltage applied to it. As can be seen in the figure, the characteristics are nonlinear.

This means that even with a small change in voltage of a few tenths of a volt, the current can change several times. However, when working with LEDs, they usually use the most linear section (the so-called working region) of the current-voltage characteristic, where the current does not change so sharply. Most often, manufacturers indicate in the LED characteristics the position of the operating point, that is, the voltage and current values ​​at which the declared brightness is achieved.

The characteristics presented above were obtained for light-emitting diodes connected in the forward direction. That is, the negative pole of the power supply is connected to the cathode, and the positive pole is connected to the anode

Calculation of a resistor for an LED

Calculating the resistor for an LED is a very important point before connecting the LED to the power source. After all, how the LED will work depends on this. If the resistor has too little resistance, the LED may fail (burn out), and if the resistance is too high, the LED will emit weak light. The resistor for the LED is calculated using the following formula:

• R = (V S – V L) / I
• V S – power source voltage (V).
• V L – LED supply voltage (usually 2 volts and 4 volts for blue and white LEDs).
• I – LED current (for example 10 mA = 0.01 A or 20 mA = 0.02 A)

Make sure that the electrical current you select is less than the maximum the LED is rated for. Convert this value from milliamps to amperes. Thus, the result of the calculation will be the resistance value of the resistor in ohms (Ohm). If the calculated resistor value does not match the standard resistor value, you must select the next higher value.

However, you may initially want to choose a slightly higher resistance, to save electricity, for example. But we must remember that the LED radiation in this case will be less bright. If the power supply voltage = 9 Volts and you have a red LED (VL = 2V), the required current is I = 20 mA = 0.02A, R = (9V – 2V) / 0.02A = 350 Ohms. You must select a resistor with a resistance of 390 Ohms (nearest larger value).

Flashing LEDs

Flashing LEDs look like regular LEDs, they can blink on their own because they contain a built-in integrated circuit. The LED flashes at low frequencies, usually 2-3 flashes per second. Such trinkets are made for car alarms, various indicators or children's toys. LED alphanumeric indicators are now used very rarely; they are more complex and more expensive than liquid crystal ones. Previously, this was practically the only and most advanced means of display; they were even installed on cell phones.

This article will talk about calculation of current limiting resistor for LED.

Calculation of a resistor for one LED

To power one LED, we need a power source, for example two AA batteries of 1.5V each. Let's take a red LED, where the forward voltage drop at an operating current of 0.02 A (20 mA) is equal to -2 V. For conventional LEDs, the maximum permissible current is 0.02 A. The LED connection diagram is shown in Fig. 1.

Why do I use the term "forward voltage drop", not the supply voltage. But the fact is that LEDs do not have a supply voltage parameter as such. Instead, the voltage drop characteristic of the LED is used, which means the amount of voltage the LED outputs when the rated current passes through it. The voltage value indicated on the packaging reflects the voltage drop. Knowing this value, you can determine the voltage remaining on the LED. This is the value we need to use in our calculations.

The forward voltage drop for various LEDs depending on the wavelength is presented in Table 1.

Table 1 - LED characteristics

The exact value of the LED voltage drop can be found on the packaging for this LED or in the reference literature.

R = (Un.p – Ud)/Id = (3V-2V)/0.02A = 50 Ohm.

• Un.p – supply voltage, V;
• Ud — forward voltage drop across the LED, V;

Since there is no such resistance in the standard series, we select the nearest resistance from the nominal series E24 upward - 51 Ohms.

To guarantee long-term operation of the LED and to eliminate errors in calculations, I recommend using not the maximum permissible current - 20 mA, but a little less - 15 mA.

This reduction in current will not in any way affect the brightness of the LED for the human eye. In order for us to notice a change in the brightness of the LED, for example, by 2 times, we need to reduce the current by 5 times (according to the Weber-Fechner law).

As a result, we get the calculated resistance of the current-limiting resistor: R = 50 Ohms and power dissipation P = 0.02 W (20 mW).

Calculation of a resistor for series connection of LEDs

In the case of calculating a resistor for a series connection, all LEDs must be of the same type. The LED connection diagram for a serial connection is shown in Fig. 2.

For example, we want to connect to a 9 V power supply, three green LEDs, each 2.4 V, operating current - 20 mA.

The resistance of the resistor is determined by the formula:

R = (Un.p – Ud1 + Ud2 + Ud3)/Id = (9V - 2.4V +2.4V +2.4V)/0.02A = 90 Ohm.

• Un.p – supply voltage, V;
• Uд1…Uд3 — forward voltage drop across the LEDs, V;
• Id – operating current of the LED, A.

We select the nearest resistance from the nominal series E24 upward - 91 Ohms.

Calculation of resistors for parallel-series connection of LEDs

Often in practice we need to connect a large number of LEDs, several dozen, to a power source. If all the LEDs are connected in series through one resistor, then in this case the voltage at the power source will not be enough for us. The solution to this problem is a parallel-series connection of LEDs, as shown in Fig. 3.

Based on the power supply voltage, the maximum number of LEDs that can be connected in series is determined.

Fig. 3 – LED connection diagram for parallel - serial connection

For example, we have a 12 V power supply, based on the voltage of the power supply, the maximum number of LEDs for one circuit will be equal to: 10V/2V = 5 pcs, taking into account that the voltage drop across the LED (red) is 2 V.

Why we took 10 V and not 12 V is due to the fact that there will also be a voltage drop across the resistor and we must leave somewhere around 2 V.

The resistor resistance for one circuit, based on the operating current of the LEDs, is determined by the formula:

R = (Un.p – Ud1 + Ud2 + Ud3+ Ud4+ Ud5)/Id = (12V - 2V + 2V + 2V + 2V + 2V)/0.02A = 100 Ohm.

We select the nearest resistance from the nominal range E24 upward - 110 Ohms.

The number of such chains of five LEDs connected in parallel is practically unlimited!

Calculation of a resistor when connecting LEDs in parallel

This connection is not desirable and I do not recommend using it in practice. This is due to the fact that each LED has a technological voltage drop, and even if all the LEDs are from the same package, this is not a guarantee that their voltage drop will be the same due to the production technology.

As a result, one LED will have more current than the others and if it exceeds the maximum permissible current, it will fail. The next LED will burn out faster, since the remaining current will already pass through it, distributed among other LEDs, and so on until all the LEDs fail.

This problem can be solved by connecting its own resistor to each LED, as shown in Fig. 5.

LEDs today have found application in almost all areas of human activity. But, despite this, for most ordinary consumers it is completely unclear why and what laws apply when operating LEDs. If such a person wants to arrange lighting using such devices, then many questions and searching for solutions to problems cannot be avoided. And the main question will be - “What kind of thing are these resistors, and why do LEDs need them?”

What is a resistor and its purpose?

A resistor is one of the components of the electrical network, characterized by its passivity and, at best, characterized by resistance to electric current. That is, Ohm’s law must be valid for such a device at any time.

The main purpose of the devices is the ability to vigorously resist electric current. Thanks to this quality, resistors are widely used if necessary, artificial lighting devices, including the use of LEDs.

Why is it necessary to use resistors in the case of LED lighting?

Most consumers know that an ordinary incandescent light bulb produces light when directly connected to any power source. The light bulb can work for a long time and burns out only when the filament heats up excessively due to the supply of too high voltage. In this case, the light bulb, in some way, implements the function of a resistor, because the passage of electric current through it is difficult, but the higher the applied voltage, the easier it is for the current to overcome the resistance of the light bulb. Of course, it is impossible to put such a complex semiconductor part as an LED and an ordinary incandescent light bulb on the same level.

It is important to know that the LED is this is an electrical device, for the operation of which it is not the current strength itself that is preferable, but the voltage available in the network. For example, if a voltage of 1.8 V is selected for such a device, and 2 V comes to it, then most likely it will burn out - if the voltage is not reduced in time to the level required by the device. It is precisely for this purpose that a resistor is required, through which the power source used is stabilized so that the voltage supplied by it does not damage the device.

In this regard, it is extremely important:

• decide what type of resistor is required;
• determine the need to use an individual resistor for a specific device, which requires calculation;
• take into account the type of connection of light sources;
• planned number of LEDs in the lighting system.

Video: Why resistors are needed

Connection diagrams

With a sequential arrangement of LEDs, when they are located one after another, one resistor is usually enough, if you can correctly calculate its resistance. This is explained by there is the same current in an electrical circuit, in each location where electrical appliances are installed.

But in the case of parallel connection, each LED requires its own resistor. If we neglect this requirement, then all the voltage will have to be pulled by one, the so-called “limiting” LED, that is, the one that needs the lowest voltage. He will fail too quickly, in this case, voltage will be applied to the next device in the circuit, which will suddenly burn out in the same way. This turn of events is unacceptable; therefore, in the case of parallel connection of any number of LEDs, the use of the same number of resistors is required, the characteristics of which are selected by calculation.

Video: Parallel connection of LEDs

Calculation of resistors for LEDs

With a correct understanding of the physics of the process, calculating the resistance and power of these devices cannot be called an impossible task that an ordinary person cannot cope with. To calculate the required resistor resistance, the following points must be taken into account:

Video: Selecting a resistor for an LED

Calculation of resistors using a special calculator

Usually, the calculation of the resistance of such devices required for any LED is carried out using calculators specially designed for this purpose. Such calculators, convenient and highly efficient, do not need to be downloaded and installed from somewhere - it is quite possible to calculate a resistor online.

Resistor calculator allows for high precision determine the required power and the resistance value of the resistor installed in the LED circuit.

To calculate the required resistance, you need to enter the following into the appropriate lines of the online calculator:

• LED supply voltage;
• LED rated voltage;
• rated current.

Next, you need to select the connection diagram used, as well as the required number of LEDs.

After pressing the corresponding button, the calculation is performed and The received calculated data is displayed on the monitor screen, with the help of which you can later organize artificial LED lighting without much difficulty.

Also, online calculators have a certain database containing data about LEDs and their parameters. The possibility of calculation is presented:

• device rating;
• color marking;
• current consumed by the circuit;
• dissipated power.

A person who is not well versed in electrical engineering and physics will, in most cases, not be able to independently calculate devices for LEDs. For this reason, carrying out calculations using a functional and convenient online calculator - invaluable help for ordinary people who do not know the methods of calculations using physical formulas.

Most well-known manufacturers of LEDs and strips created on their basis, on their official websites They also post their own online calculator, with the help of which you can not only select the required resistors and LEDs, but also calculate the parameters of the current devices used in various operating modes with variable values ​​of current, temperature, applied voltage, etc.